1. Field of the Invention
The present invention relates to a plasma processing method and apparatus that produces a plasma by the energy of a high-frequency wave, such as a microwave, etc. and processes a workpiece, such as a semiconductor wafer, with the plasma.
2. Description of the Related Art
Semiconductor device fabricating processes include a plasma processing process that processed semiconductor wafers (hereinafter referred to simply as "wafers") with a plasma. FIG. 12 shows a known microwave plasma processing system for carrying out such a plasma processing process. This known microwave plasma processing system has a vacuum vessel 9, a work support table 91 placed in the vacuum vessel 9 to support a wafer W thereon, and a plane slot antenna 92 disposed in an upper region of the interior of the vacuum vessel 9. Microwaves radiated by a microwave power generator 93 is fed through a waveguide 94 to the antenna 92, the microwaves are fed from the antenna 92 to produce a plasma by ionizing a processing gas supplied from a gas supply pipe 95, and a film is deposited on the surface of the wafer W or the surface of the wafer W is etched with the plasma. In FIG. 12, indicated at 96 is a quartz window.
A load matching device 97 is placed on the waveguide 94 to make conjugate the impedance of a section of the waveguide 94 between the load matching device 97 and the plasma and the impedance of a section of the same between the load matching device 97 and the microwave power generator 93 to suppress reflection of the microwave from the plasma.
The electron density of the plasma is one of factors dominating the condition of the process for processing the wafer W. When a gas that does not produce negative ions is used, electron density is substantially equal to plasma density. When a gas that produces negative ions is used, negative ion density increases as electron density decreases. Therefore, it is desirable for stable processing to keep electron density constant during the process. However, it is difficult to know electron density and hence it is not easy to control electron density.
The condition of a plasma is dependent on the microwave power and pressure in the vacuum vessel 9. Therefore, microwave utilization factor in using a microwave for producing a plasma decreases even if the impedances are matched by the load matching device 97 because some condition of the plasma increases the amount of the microwave reflected by the plasma toward the microwave power generator 93 and the reflected microwave generates heat due to dielectric loss or ohmic loss.
The present invention has been made in view of such problems and it is therefore an object of the present invention to enable monitoring the electron density of a plasma by measuring the reflection coefficient of high-frequency waves, such as microwaves or RF waves, radiated by a power generator.